Apply (State s) is confusing to the extreme ... my variable names doesn't help either

gistfile1.hs

instance Apply (State s) where
  -- Apply f ⇒ (<*>) :: f (a -> b) -> f a -> f b
  -- Apply (State s) ⇒ (<*>) :: State s (a -> b) -> State s a -> State s b
  --sab <*> sa = (\f → f <$> sa) <$> sab
  sab <*> sa =
    let
      fsab = runState sab -- ∷ s → (a → b, s)
      fsa  = runState sa  -- ∷ s → (a, s)
      apply s =
        let
          as  = fsa s
          a   = fst as
          abs = fsab (snd as)
          ab  = fst abs
          s2  = snd abs
        in
         (ab a, s2) 
    in
   State { runState = apply }